Plasma verapamil levels were measured using a highly sensitive assay employing high pressure liquid chromatography and fluorometric detection in all patients with hypertrophic cardiomyopathy on chronic oral verapamil therapy. There was marked interpatient variability in verapamil levels at each dosage and no differences between clinical responders, nonresponders, and patients with serious cardiac side effects related to verapamil. Plasma verapamil levels, however, do have limited usefulness for monitoring therapy in patients with hypertrophic cardiomyopathy.